When humanity seeks what separates us from other animals, and even our ancestors, we usually consider the size of our brains, with a little help from opposable thumbs. However, our brain's shape is also unusual among primates, and a team of scientists think it was the appearance of a more spherical brain, rather than any increase in size, that enabled the cultural explosion that preceded humanity's conquest of the world.

The size of the human brain grew dramatically over millions of years. The accompanying rise in intelligence can be tracked in the sophistication of tools accompanying expanding skulls through the fossil records. Yet anthropologists have been puzzled by the fact that there was a long pause between when brains apparently stopped growing and the great flowering of human culture and technology.

Dr Simon Neubauer reexamined 20 skull casts from early Homo sapiens. Along with colleagues at the Max Planck Institute of Evolutionary Anthropology, Neubauer reports in Science Advances that “300,000 years ago, brain size in early H. sapiens already fell within the range of present-day humans.”

That's not the end of the story, though. “Brain shape, however, evolved gradually within the H. sapiens lineage, reaching present-day human variation between about 100,000 and 35,000 years ago,” the paper continues. This timing is significant because it matches the rise of many of the behaviors we consider specifically human, including exploration beyond Africa.

As the authors note, brains don't fossilize well, so we have to estimate the shape of our ancestors' thinking organ from the inside of their skulls. Nevertheless, we know our own brains are globular with “steep frontal, bulging parietal, and enlarged rounded cerebellar areas.” The brains of other living great apes are shaped very differently, and the skulls of other extinct Homo species are also quite unlike our own.

Neanderthals, for example, had braincases slightly larger than our own, relative to body size, and a much longer axis from front to back. The different shape appears to have emerged early in the childhood of Neanderthals and modern humans, at the time when the brain develops most rapidly. Genetic comparisons between modern humans and Neanderthals provide an indication of the changes that drove these different paths.

The earliest Homo Sapiens (1) had skulls shaped halfway between those of Homo erectus and Neandertals. Somewhere along the way, we diverged from the Neanderthals in this, and it may have been crucial to our success. Neubauer et al. Science Advances.

Human skulls fossilized in Morocco around 300,000 years ago not only had similar-sized braincases to our own, but distinctly modern-looking faces and jaws. Yet the back of their skulls were quite different.

The expansion of certain brain regions, particularly the cerebellum, associated with language and social cognition among other features may have enabled major advances that occurred at the same time. These include the use of pigments, adoption of ornaments, and the beginnings of symbolic art that did much to make us who we are.